For many years the changing of gear ratios in a mechanical device has been accomplished by disengaging the torque from the transmission system, usually with a clutch.
Attempts also have been made to change gear ratios by randomly forcing a plurality of "keys" between closely spaced teeth of internally-toothed gears. This brute force approach also involves a momentary disengagement of torque from the transmission and some slippage may occur when power is being transferred from one set of keys to another set of keys, i.e., before the other set is randomly engaged. This approach also may cause rapid wearing and/or breakage of both the keys and the internal teeth, as well as binding and/or lockup of gears.
Different non-gear designs have been used for the purpose of changing the ratio while providing continuous torque. One such design is a double pulley arrangement constructed of metal elements that drive a V-belt. This design is presently used in snowmobiles, but belt wear resulting in slippage and/or belt breakage has been a problem.
Another design uses rolling elements between an inner input gear and an outer output gear arranged much like an epicyclic gear train. Although the rolling elements can continuously change the gear ratio, they are torque-limited because they rely on the coefficient of static friction to transmit forces between the rolling elements.